There are a couple of things that could cause this, unfortunately there is no reset for this television other than unplugging it. It will have an internal failure at this point. I will try to explain the possibilities the best I can.
First you could have a failure in the power supply, we have seen some capacitors fail in your model and cause funky problems like this. If this is the case you could be looking at somewhere around $175-$200 to fix the set. IF the power supply had to be replaced you could be looking at $250-300
Next could be the audio portion of the main board, this would not be good news because you could be looking at $300-$400 for the repair
Any of these could cause your problem, but it will require troubleshooting to determine the exact cause of your problem. If you have experience, and test equipment and would like to attempt some troubleshooting I will be happy to assist you, however if you do not have equipment or experience then you would need a repairman at this time to get an exact diagnosis. I am sorry that I don't have a quick fix or easy answer for you on this one, it is definitely an internal failure.
If you would like any additional information or have more questions please don't hesitate to ask!
Please never accept an answer until you are 100% satisfied and feel you can leave positive feedback. If I have answered your questions and met your needs then please press the green "accept" button. Otherwise please feel free to ask anything else to get your problem resolved. Positive feedback is always appreciated!